Zhijun Li and Tian He

Computer Science and EngineeringUniversity of Minnesota

BEST Paper Award @ MobiCom 2017

Tian He @ UMN

Wireless is Everywhere

<wigle.net>

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Tian He @ UMN

1.8B

… and Increases Rapidly

Source: ABI Research

~4B

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Gartner predicts 20 billion IOT devices by year 2020

Source: Navigant Research

130M

Tian He @ UMN

… also Diversifies Quickly

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Tian He @ UMN

Coexist and CollaborateLANNetwork of

“Things”ThermostatSmoke detectorHumidity detectorMany more

PAN

WAN

Smart Hospital

Tian He @ UMN

Objective: Ubiquitous Connectivity

Q: How to Interconnect/bridge them?

PAN

LAN

PAN

WAN

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Tian He @ UMN

MutiRadio Gateway

Bridging Wireless Tech: Gateway

�Extra HW/deployment cost�Traffic overhead into/out of the gateway�Pre-deployment, unsuitable for ad hoc/mobile

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Tian He @ UMN

The New Paradigm: CTCCross-Technology Communication (CTC) enabling heterogeneous devices talks directly!

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Tian He @ UMN

What CTC can Achieve: Low Cost

A WiFi AP controls all smart home ZigBee-enabled devices in one hop without gateway

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Tian He @ UMN

What CTC can Achieve: Mobility

Direct Communication among mobile IOT devices without pre-deployed gateways

Battle FieldVANET

Inventory Tracking Wearables

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Tian He @ UMN

Support Explicit channel coordination

� Extends local mechanisms globally across wireless tech.� Global RTS/CTS Reservation.� Global Time Division Multiple Access (TDMA)

What CTC can Achieve: Coordination

Time

Wifi

No traffic next 20ms

ZigBee Wifi

I need next 200ms

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Tian He @ UMN

WEBee

Physical-Layer Cross-TechnologyCommunication

WEBee: WiFi Emulated ZigBee

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Tian He @ UMN

Outline

� The State of the Art

� The Design

� Implementation & Evaluation

� Extensions

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14Tian He @ UMN 14

The State of the Art

Tian He @ UMN

Packet-level CTCUsing packet length, gap, or offset

A packet can only express a few bits

A Brief History of CTC Research

Mobicom2009

INFOCOM2013

MobiCom2015

MobiCom2017

ESENSEPacket Length

K. Chebrolu

GSenseGap Length

X. Zhang

HoWiESComb. Packet

LengthY. Zhang

FreeBeeBeacon Interval

S. Kim

bps

PHYCTC

100sKbps

bps bps

16,000X Faster!!! WE ARE HERE

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timeEner

gy(R

SSI)

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1

Tian He @ UMN

Physical-Level CTCBy emulating receiver’s RF waveform, A packet achieves Rx’s maximum rate

A Brief History of CTC Research

Mobicom2009

INFOCOM2013

MobiCom2015

MobiCom2017

bps

PHYCTC

100sKbps

bps bps

16,000X Faster!!! WE ARE HERE

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I/Q

RSS

I

time

QuadratureIn-phase

11 1 0

……

17Tian He @ UMN 17

The Design

Tian He @ UMN

The Key Idea of WEBee

WiFi Sender

Emulated ZigBee Signals

ZigBeeReceiver

WiFi Frame with selected Payload

ZigBee Frame

Ignore IgnoreZigBeeCompatible Signal

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Tian He @ UMN

Challenges of Physical-Layer CTC

Modulation: QAM, OFDMRate: ~54Mbps (802.11g)Distance：300mTx：26dBmSensitivity：-80dBm

Modulation : OQPSK, DSSSRate: ~250KbpsDistance：100mTx：0dBmSensitivity：-97dBmWithout translator

Key InnovationSignal

Emulation@

Wi-Fi Sender

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Tian He @ UMN

How Wi-Fi Transmits

Wi-FiFrame

Modulation(QAM)

Scrambler ConvolutionalEncoder Interleaver

Channel Coding

PilotInsertion IFFT Cyclic

Prefixing

Post-QAM ProcessingWi-Fi Signal

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Coded Bitspayload

Tian He @ UMN

Emulation via a Reverse Path

Wi-FiFrame

Scrambler ConvolutionalEncoder Interleaver

PilotInsertion IFFT Cyclic

Prefixing

Modulation(QAM)

TransmissionEmulation

Channel Coding Emulation

Post-QAM Emulation

QAMEmulation

ZigBee SignalsTo be Emulated

Coded Bitspayload

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Tian He @ UMN

QAM Emulation

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Mimic

RF Signal Spectrogram in the Air

QAM/OFDM signals OQPSK signals

Tian He @ UMN

� Minimizing emulation distortion in the time-domain is equivalent to minimizing the total deviation of frequencycomponents (based on Parseval’s theorem)

The Closest Wi-Fi QAM Points

Minimizing Emulation Distortion

Desired Time-domainZigBee Signals

FFT QAMEmulation

CorrespondingFrequency-domain

Components

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Tian He @ UMN

Parallel CTC

Link-level reliability

Other Technical ChallengesReverse channel coding

Pilot/Null avoidance

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For details, refer to the paper

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Innovative Features of WEBee

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Dual-Standard Compliance

A part of WiFi frame is a Zigbee frame

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No change in Sender’s Hardware/Firmware

Send a normal WiFi Frame

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No change in receiver’sHardware/firmware

Receive a normal ZigBee Frame

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Combine the advantages of two technologies.

Tx：26dbmSensitivity：-65dbm

Rx：0dbmSensitivity：-97dbm

Big Mouth Good Ear

WEBee has a longer range than Wi-Fi!

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Support parallel CTC in one WIFI Packet

Simultaneous Unicast

20MHZ

2MHZ

2MHZ

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Support high mobility and duty cycled operations

Longer Range, better mobility Low Power Listening

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An 16,000x faster CTC with 99.9% reliability

For details referring to our mobicom 2017 paper

Tian He @ UMN

Implementation & Evaluation

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Tian He @ UMN

System Implementation

WiFi(Commodity)

AR9485

ZigBee(USRP)

N210

ZigBee(Commodity)

CC2420

WEBee @USRP

N210

WEBee@ SmartphoneLGNexus5

WEBee@Laptop

AR2425

ReceiverSender

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RXWi-Fi Frame

RX ZigBee Frame

RX ZigBee Frame

Tian He @ UMN

Experimental Setting� Lab� Hallway� Outdoor

Experiment setting

10,000 runs each trial

• Varying distances • Varying Tx Power• Varying packet length• Varying content• Varying duty-cycle• Varying Mobility

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Tian He @ UMN

Symbol Error Ratio vs Data RateFreeBee [Mobicom ’15]

7.5bps@99%

WEBee [Mobicom ’17]

63,000bps@99%

~8,000xSpeedup*250kbpstheoretically

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Tian He @ UMN

Frame Reception Ratio

~50%

WEBee(USRP) è ZigBee

WEBee(WiFi) è ZigBee

Rec

ept.

Rat

io (%

) 20

4060

8010

00

A frame fails with one symbol error

WEBee(Commodity)

AR2425

WEBee(USRP)

N210

ZigBee

CC2420

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Tian He @ UMN

Reliability after Retransmission

99%after6retransmissions

Less re-TX with Repeated preamble and coding

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Tian He @ UMN

Parallel CTC

Withtwochannels,WEBee vs.FreeBee =~16,000x

Channel I Channel II39

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WEBee in Action

Tian He @ UMN

WEBee Demo

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Tian He @ UMN

Application: Smart Light Control

GE Gateway Solution WeBee Solution

Vs.

� You can control ZigBee Smart Bulb with WEBee

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Tian He @ UMN

Release: WEBee Payload Generator V1.o

You can conduct cross-technology research using WEBee Generator!Available at: http://tianhe.cs.umn.edu/CTC

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Possible New Topics:

• Channel Access Control• Coordination• Neighbor Discovery• Multi-Technology Routing• CTC Time Synch.• CTC Localization • …..

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Extension

Tian He @ UMN

Question:Whether Signal Emulation

is a generic technology?

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Tian He @ UMN

BlueBee: BlueTooth to ZigBee [SenSys ’17]

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Tian He @ UMN

LTEBee: LTE (band 7) to ZigBee� LTE smartphone controls a CC2530 ZigBee Blub directly

after 7-layers channel coding and 6-step modulation.

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Tian He @ UMN

Conclusion� WEBee is the first physical-layer CTC design, a

paradigm shift with a significantly higher throughput, while requiring no change of HW.

� Our work indicates Signal Emulation is a generic technology to build light-weight SDR, striking a delicate balance between flexibility and cost.

� Our work brings a surge of opportunities to expand many local wireless mechanisms globally (e.g., coordination, discovery, etc.) across technologies.

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Tian He @ UMN

CTC Technical Support is available at

http://tianhe.cs.umn.edu/CTC

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Can support two-way

MasterNode

Slavenodes

Duty-Cycle ModeAlways On Mode

Wake

Long Range

Tian He @ UMN

� Wireless technologies share unlicensed ISM bands, offering opportunity for cross-tech. communication

Opportunity for Cross-Tech.-Comm.

<ni.com>

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ChannelCenter Frequency (MHz)

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Tian He @ UMN

Channel Mapping� OFDM Has 64 subcarrier, 48 data, 12 null and 4 pilot

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Tian He @ UMN

Four-to-one emulation� Four Wi-Fi symbols are used to emulate one ZigBee symbol

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Tian He @ UMN

Link Layer Reliability � Repeated preamble and hamming coding

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